By the time you realzed what was happening, figgured out which motor to eject, re-orintated the machine, started the tricopter program, got the gyros centered (while falling through the air) then started flying it again - your quad woud be about 30 feet underground.

I agree with moparren. It would be difficult to do just as an intentional trick let alone as a fail safe system. Not worth the effort. Some type of octocopter configuration would offer better redundancy. I've seen those fly with a motor or two out.

Or just buy better electronics than what the HobbyKings of the world provide.